Digital picture data, for example, are exorbitant in data volume, so that, if the data is to be recorded or transmitted directly, a recording medium or a transmission medium of an extremely large capacity is required. Thus, in general, picture data are compression-coded for diminishing the data volume before recording or transmission.
Among the systems for compression-coding a picture, there are a JPEG (Joint Photographic Experts Group) system, as a compression encoding system for a still picture, and an MPEG (Moving Picture Experts Group) system, as a compression encoding-system for a moving picture.
For example, the encoding and decoding of picture data in accordance with the JPEG system are performed as shown in FIG. 1.
FIG. 1A shows an illustrative structure of a conventional JPEG encoding device.
Picture data to be encoded are input to a blocking circuit 1 which then divides the input picture data into plural blocks each composed of 8×8=64 pixels. The blocks obtained in the blocking circuit 1 are routed to a discrete cosine transform (DCT) circuit 2. This DCT circuit 2 applies DCT processing to each block from the blocking circuit 1 to transform the block into a sun total of 64 DCT coefficients comprised of a sole DC (direct current) component and 63 frequency (AC (alternating current)) components. The block-based 64 DCT coefficients are routed from the DCT circuit 2 to a quantization circuit 3.
The quantization circuit 3 quantizes DCT coefficients in accordance with a preset quantization table to route the results of quantization, referred to below as quantized DCT coefficients, along with the quantization table used for quantization to an entropy encoding circuit 4.
FIG. 1B shows an instance of a quantization table used in the quantization circuit 3. In the quantization table, the quantization steps are set in general in order to take characteristics of the human visual system into account so that low frequency DCT coefficients, which are more crucial, are quantized finely, whereas high frequency DCT coefficients, which are less crucial, are quantized only coarsely. This suppresses deterioration of the picture quality of a picture to assure efficient compression.
The entropy encoding circuit 4 applies entropy encoding processing, such as Huffman encoding, to the quantized DCT coefficients, from the quantization circuit 3 and appends the quantization table from the quantization circuit 3 to output the resulting encoded data as JPEG encoding data.
FIG. 1C shows the structure of an instance of a conventional JPEG decoding device 10 used for decoding the encoded data output by a JPEG encoding device 5 of FIG. 1A.
The encoded data are input to an entropy decoding circuit 11, which then separates the encoded data into entropy encoded quantized DCT coefficients and the quantization table. The entropy decoding circuit 11 entropy decodes the quantized entropy encoded DCT coefficients to route the resulting quantized DCT coefficients along with the quantization table to a dequantization circuit 12. The dequantization circuit 12 dequantizes the quantized DCT coefficients from the entropy decoding circuit 11, in accordance with the quantization table from the entropy decoding circuit 11, to send the resulting DCT coefficients to an inverse DCT circuit 13. The inverse DCT circuit 13 applies inverse DCT processing to the DCT coefficients from the dequantization circuit 12 to send the resulting 8×8 pixel decoded block to a deblocking circuit 13. The deblocking circuit 13 deblocks the decoded blocks from the inverse DCT circuit 13 to derive an output decoded picture.
With the JPEG encoding device 5, shown in FIG. 1A, the data volume of the encoded-data can be diminished by increasing the quantization step of the quantization table used in quantizing the block, thereby realizing high compression.
However, if the quantization step is increased, the so-called quantization error is also increased to deteriorate the picture quality of the decoded picture obtained in the JPEG decoding device 10 of FIG. 1C. That is, blurring, block distortion or the mosquito noise etc are produced appreciably in the decoded picture.
Thus, if the data volume of the encoded data is to be diminished but nevertheless the picture quality of the decoded is not to be deteriorated, or if the data volume of the encoded data is maintained as the picture quality of the decoded picture is to be improved, some processing or other for improving the picture quality needs to be carried out subsequent to JPEG decoding.
However, if processing for improving the picture quality is to be performed subsequent to JPEG decoding, the processing is complicated to prolong the time until the decoded picture is ultimately produced.